I recently read a book that suggests using soil blocks for starting seeds. What are the advantages and disadvantages of soil blocks over the more popular seed starting methods? I am mainly interested in using the seed blocks to start vegetables.

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Probably the biggest disadvantage of using soil blocks is the up-front cost. If you want to get the biggest advantage, you'll want to have the ability to transplant from smaller blocks to larger blocks. For example, you'd start tomatoes in 2" blocks and then transplant up to 4" blocks. I've seen a number of people recommend the little tongs/tweezers, which would be an extra expense. (I don't own them, but I could see how they might make handling blocks a bit faster.) Buying all of these tools (3/4", 2", 4", tongs) at once will probably cost $200 or more.

A minor disadvantage is that all of the soil block recipes I've seen use peat. (I'd like to reduce my use of peat for sustainability purposes, but I'm hoping that reducing the use of plastic planting containers is a net gain.)

You'll want to mix your own potting soil. (I do this anyway, so there's no net change for me.) I suppose that you may be able to buy pre-mixed block soil, but it's fairly easy to do and you get to have control over the ingredients.

You'll want to set up a good watering system. I've done a couple of seasons of blocks without a good watering system, but I'm working on something for the upcoming planting season. The two problems related to watering are: (1) spraying the blocks with a hand sprayer is tedious and could lead to problems with fungal disease, and (2) if you pour water into the tray with a watering can (i.e. so that the stream of water makes contact with the blocks as you pour), you'll erode the blocks. Setting the blocks on a mesh screen so you can bottom-water seems like it will be ideal. I'm calling this a "drawback" because it's yet another bit of upfront setup you have to deal with.

It's not really a drawback, but I find that when I get in the groove of making blocks I will enthusiastically make a lot of blocks and then end up with a few too many plants. (A sure symptom of this is when you catch your friends saying, "Quick! Hide, he's going to try to give us some more lettuce in those weird little dirt bricks!")

Lastly, there's a learning curve: consistently making good blocks takes some practice. You've got to get the hang of mixing the soil to the proper consistency and then form the blocks correctly. It's not difficult, but it does take a bit of practice.

Benefits:

No containers.

This reduces an ongoing cost and hassle.

There's nothing to clean, nothing to throw away.

It's a big time saver.

Once you get the hang of making blocks, you can make a lot of blocks very quickly. (Don't be discouraged by your first couple of block making sessions. Stick with it and it will work out.)

"Potting up" is really fast. Just drop the small block into the big block. No pricking out, no trying to squeeze plants out of little plastic containers.

Transplanting into the garden is really fast for the same reason.

You can make efficient use of space: if you use a heating mat, you can put a whole mess of 3/4" blocks onto the mat to germinate the seeds and then "pot up" the 3/4" blocks into 2" blocks.

It's gentler on the plants.

No root shock when transplanting.

Plants don't get root bound. The roots will be air pruned when they reach the edge of the blocks.

No pricking out, no squeezing/pulling plants out of containers.

Avoiding all that shock keeps plants growing steadily without setbacks. In theory this should give you higher quality plants that mature faster and yield better.

The block makers from Ladbrooke are high quality tools. I think that with proper care they're likely to last a lifetime.

My suggestion to get started slowly would be to start with the 1.5" or 2" blocks. These are good for starting a large variety of plants: spinach, lettuce, broccoli, cabbage, etc. Then you can add either the 3/4" blocks if you want to be able to germinate larger numbers of seeds at once, or the 4" blocks if you want to be able to start plants that should be potted up to a larger size before transplanting in the garden (e.g. tomatoes, peppers) or those that are large to begin with (e.g. melons, pumpkins).

By the way: an old cookie sheet can hold about 300 3/4" blocks, and 35 2" blocks.